PROJECT SUMMARY Tuberous Sclerosis Complex is a neurodevelopmental disorder caused by mutations in the TSC1 or 2 genes that encode negative regulators of mTOR complex 1 signaling. TSC is associated with a high prevalence of autism spectrum disorder (ASD) and other neuropsychiatric conditions, which are debilitating for patients and caregivers. Despite their prevalence in TSC, relatively little is known about the neurobiology of these manifestations including the cell types responsible. We propose that a core aspect of ASD, repetitive, inflexible patterns of behavior, is caused by synaptic changes in the basal ganglia, a brain region responsible for the selection and learning of appropriate actions. Here we will investigate this in the context of TSC by determining how mutations in Tsc1 affect the cellular physiology and behavioral output of neurons comprising key basal ganglia circuits. To isolate specific cell types, we will use genetic mouse models in which Tsc1 is selectively deleted from defined cell populations. The experiments in Aim 1 will determine how Tsc1 loss affects synaptic transmission and plasticity in the two classes of striatal projection neurons that initiate the primary output pathways of the basal ganglia. We will test the idea that increased cortical synaptic drive of direct pathway striatal neurons leads to altered learning and increased propensity for motor habit formation. Striatal activity is dynamically regulated by dopamine signaling, which exerts powerful control over behavior. In Aim 2, we will determine how selective deletion of Tsc1 from dopamine neurons affect their physiology and output. We will test the hypothesis that loss of Tsc1 causes hypofunctional striatal dopamine signaling leading to impaired cognitive flexibility in reversal learning tasks. This strategy represents a key step towards dissecting the cellular and circuit basis of TSC, and may ultimately inform new therapeutic strategies for this and related ASDs.